1. Field of the Invention
The present invention relates to phase-locked loops and, more specifically, to phase and frequency detection with little static phase error in phase-locked loop systems.
2. Discussion of Related Art
Phase-locked loops (“PLLs”) are widely used in modern electronic devices due to their capability of generating an internal feedback clock signal that is phase aligned with an external reference clock signal. PLLs have been utilized in various applications including, for example, cross-chip communications, signal synchronization, data recovery, and frequency modulation.
A typical PLL integrates a phase and frequency detector (“PFD”), a charge pump, a low pass filter, and a voltage-controlled oscillator (“VCO”) in a negative feedback closed-loop configuration. The PFD in a PLL receives a reference clock signal and an internal feedback clock signal and generates two pulsed signals based on the detected phase difference between the reference clock and internal feedback clock signal. These pulsed signals drive the charge pump to adjust the control voltage provided to the VCO, thereby changing the frequency of the signal, output by the VCO. In current PFD implementations, the level of both pulsed signals generated by the PFD may be set to a high logic level during a period when no charge should be injected by the charge pump. In such an instance, if the source and sink current sources are perfectly matched, the net charge injected by the charge pump is ideally zero. Actual charge pump current sources, however, often exhibit some mismatch, causing the internal feedback clock signal generated by the VCO to shift in phase from its ideal location. Non-ideal phase shift attributed to mismatched charge pump current sources is called static phase error. Static phase error may be reduced by minimizing the period during which the charge pump source and sink current sources simultaneously inject charge.
It is desirable to develop a novel and improved PFD that reduces static phase error and relaxes matching requirements of charge pump currents.